Wind Turbines vs. Weather Radar in Tornado Alley (Nebraska showdown)

Since 2009, two Nebraska communities have been destroyed by tornadoes, the most recent in June, 2014 (Pilger, NE). News stories covering these disasters can be found here and here.

Needless to say, accurate weather forecasting is essential for protecting life and property. But what happens when wind turbines are placed too close to a NEXRAD[1] weather radar? We may know soon enough.

NextERA's Cottonwood wind project (113.6 MW) proposes to site all of its 52 turbines within 2½ to 7 miles of the Blue Hill, Nebraska NEXRAD facility. This will be the closest of any operating turbines in the U.S. to weather stations in Tornado Alley. 

Privately, local weather service employees have expressed concern and, given Nebraska's storm history, they're right to be wary. An illustration of the project and radar station can be viewed here. 

Weather Radar and Turbine Impact

Forecasters watching for severe weather look for key indicators in the radar data including the strength of the signal reflecting off the storm cloud, the rate at which the cloud is growing (or if its decaying), and its direction/ speed. This information is critical in determining the size and intensity of the storm and the geographic area likely to be impacted should a warning be issued. 

Wind turbines within 30 nautical miles of weather radar (line of sight) can significantly compromise the radar data[2] and impair a forecaster's ability to evaluate developing weather events. 

Tracking a storm's rapidly changing behavior as it moves into the turbine obstructed radar beam (the "cone") is complex and may not be possible.  

Washington Knows Better

But not to worry. According to NOAA's Administrator, Dr. Kathryn Sullivan, concerns about NextEra's project are misplaced. In a letter to Nebraska Senator Deb Fischer, Dr. Sullivan touts an agreement signed between the Hastings, NE Weather Forecast Office (WFO) and NextEra[3] outlining mitigation steps for curtailing project operations during severe weather conditions. Shutting down the turbines will enable weather forecasters to view radar data that's uncontaminated by the moving blades.[4]

"[T]he mitigation strategy and forecaster training," wrote Dr. Sullivan, "will reduce impacts and ensure that the Cottonwood project is not a threat to the integrity of NOAA's forecast and warning operations."

Convinced? Don't be. 

For starters, the agreement is entirely voluntary and non-binding on the parties. But there's more. 

Mitigation Plan Ignores Reality

The NextEra-WFO agreement provides a sample scenario of the curtailment process as shown in the table below:

Time	Action
3:20pm	WFO forecaster identifies severe thunderstorm with tornadic potential 15-miles SW of the wind farm and on track to pass over or near the project at approximately 4:14pm;
3:21pm	Forecaster asks the Severe Weather Coordinator to contact the wind farm and request operational curtailment beginning at 4:00pm and lasting 30 minutes;
3:26pm	Severe Weather Coordinator contacts wind farm and makes request for curtailment; Wind farm agrees to curtailment and initiates shutdown;
3:56pm	Turbine shutdown complete; Forecaster interrogates storm;
4:20pm	Forecaster issues tornado warning;
4:25pm	Storm clears the wind farm;
4:30pm	Wind farm resumes turbine operations
4:40pm	Wind farm back to full operation.

It would be nice if the growth and decay of severe weather were that well known, but it's not. 

For forecasters watching a developing thunderstorm, the total time interval during which the storm might cause damage is very short, but critical, and very difficult to forecast even with full radar access. Severe weather events can form, explode into massive life-threatening storms and fully dissipate all in under an hour, making the timeline depicted in the above table utterly laughable. In the real world, the storm threat may be over before the turbine shutdown is complete. 

The area near the Cottonwood project, on average, experiences roughly 48 thunderstorm days per year (as many as 50-60+ storms) during the spring and summer months. Statewide over 11,000 hail events can be expected in the same period.[5] Are all of these events severe enough to trigger warnings? No. Can any one of these evolve into a severe storm? Yes! 

A line of storms approaching at the right angle could fall within the obstructed area of the radar beam for many, many hours. Ideally, curtailment requests should be in effect for as long as the storm threat exists, but that's not what the agreement says. The maximum duration of any shutdown is 1-hour with no more than 12-hours of curtailment enforced per year. NextEra is assured 15-30 minutes lead time for shutdowns during regular business hours. Longer time is needed on holidays, evenings, and weekends. After all, everyone knows weather slows down with the rest of us. 

Why would any experienced meteorologist agree to force-fitting severe weather forecasts into NextEra's tidy schedule? Maybe because NextEra promised to help out. 

According to Dr. Sullivan, the project's full-time wind farm operators will be trained as storm spotters. 

Seriously? 

What would they be trained to spot, a thunderstorm? By time they see the storm it's too late. If they spot a tornado, it's really too late. What about the more than 50% of Nebraska's severe weather that occurs at night and with no regard for holidays and weekends. Does anyone expect NextEra employees to make up for the loss in radar resolution? 

More Window-Dressing By Big Wind

Accepting these mitigation measures without fully understanding their effectiveness could place the lives and property of Nebraskans at risk. And what happens if/when the WFO finds its forecasting work is compromised by the project? Paragraph 15 says they can have a sit-down with NextEra. Paragraph 16 prohibits the parties from publicly commenting on the performance of the other so if the mitigation fails, we'll never know. Then there's Paragraph 19 which says the agreement terminates after 5 years but either party can end it at any time. 

This agreement is nothing more than window-dressing. If Dr. Sullivan read the agreement, and that's a big 'if', the people of Nebraska could not have been her first priority when she responded to Senator Fischer. 

Hopefully, Senator Fischer and newly elected Senator Sasse are paying attention to what's happening in their state. For those living outside of Nebraska, we should be keenly aware of how the wind industry has been allowed to degrade our public assets and place our fellow Americans at risk. 

Special thanks to meteorologist Dr. Fred Ward who helped us to better understand the difficulties of forecasting weather and the importance of radar.

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[1]   Next Generation Weather Radar (NEXRAD) system. NEXRAD is a key tool used by weather forecasters when preparing forecasts and severe weather warnings.

[2]   Turbines impact radar data in at least two ways: (a) the spinning blades can block a significant percentage of the radar beam and decrease its signal strength down range of the wind facility; and (b) the turbines reflect energy back to the radar system which can produce false storm activity. NEXRAD filters that remove clutter from the radar signal only work for stationary objects (buildings, trees and terrain).

[3]   In the agreement, NextEra is operating as Cottonwood Wind Farm, LLC.

[4]   Only two such agreements exist in the US. One involves a wind project in Kansas the other in New York State. The agreements have not been made public, to the best of our knowledge. 

[5]   Data supplied by the Warning Coordination Meteorologist at the Storm Prediction Center in Norman, Oklahoma.